Basic azomethine dyes are used extensively in the form of relatively dilute aqueous solutions in the paper, textile and leather industries. Heretofore, dyeing solutions have been prepared by the dyer from finely ground dyes. The finely ground dyes have the disadvantage of caking through the action of moisture or heat, with subsequent difficulty in dissolving the dyes. Moreover, weighing or transferring the powdered dyes is attended by unpleasant dust formation. In addition, during dissolution of the dyes and subsequent stirring, frothing often takes place leading to consequent contamination and loss.
The process of this invention has several advantages over prior art processes. The novel process leads to pure dyes that are substantially free of unreacted azo dye base and/or protonated dye salt contamination caused, inter alia, by not using acid-binding agents or by not using them in sufficient amount. Because of this freedom from azo dye base and protonated dye salt contamination, the dyes produce brighter dyeings having very good light fastness and sublimation fastness compared to dyeings made with contaminated dyes.
Further, the novel process leads to dyes substantially free of the water-insoluble carbinol form of the dye which requires an additional acid neutralization step to be converted to the desired quaternized form. The carbinol form sometimes results from alkaline neutralization of the quaternized dye salt during quaternization when an alkaline acid-binding agent is employed. Detectable amounts of the carbinol form in the final dye will of course, cut down correspondingly on dye strength and adversely affect the shade. A lowering of yield will also occur.
The novel process of this invention for making dyes of the formula ##STR2## is an improvement in the process of reacting an azo dye base (precursor) of the formula ##STR3## wherein A is phenyl or phenyl substituted by 1 to 3 groups selected from C.sub.1-4 alkyl and C.sub.1-4 alkoxy, and X.sup..crclbar. is an anion, with dimethyl sulfate in solution, and in the presence of an acid-binding agent, the improvement comprising
(i) reacting dimethyl sulfate in a molar excess of from 100% to 300% per mole of precursor, in an aqueous solution having at least 30 weight percent water and from 2.0 to 3.5 moles of magnesium oxide per mole of precursor.
The azo reactant is referred to as the "azo dye base" for convenience and simplicity. It should be understood, however, that the corresponding acid salts are also operable as reactants (precursors) and are therefore included within the scope of the invention without being referred to repeatedly.
Reaction temperatures are generally maintained at between about 60.degree. C to 120.degree. C. Following the reaction of step (i) to produce the dye solution, the dye can be isolated in dry form. Isolation of the dry dyestuff can be conducted readily at a pH below about 2.5.
An alternative to the isolation of the dry dye is to adjust the constituency of the dye solution that is prepared by the novel process outlined above, to produce preferred dye solutions. That is, to produce preferred dye solutions an additional step is needed of adjusting the constituency of the intermediate dye solutions. The preferred azomethine dye solutions made according to the process of this invention comprise:
5% to 50% by weight of dye,
10% to 95% by weight of solvent, and
0 to 40% by weight of water.
The additional process step concerns adjusting the intermediate dye solution constituency to remove organic reaction solvent and/or water, and to add the other necessary ingredients in the requisite amounts. For the sake of simplicity the term "adjusting constituency" encompasses the process step, step (ii), of adding and/or removing ingredients. It should be understood that such "adjusting" is conducted at a pH below about 2.5 and the term includes acidification to achieve such pH.
The anion X.sup..crclbar. can be derived from an inorganic or organic acid, e.g. from hydrohalic acids (Cl.sup..crclbar., Br.sup..crclbar., I.sup..crclbar.), sulfuric acid (SO.sub.4 .crclbar.) methylsulfuric acid (CH.sub.3 SO.sub.4.sup..crclbar.); from alkyl or aryl sulfonic acids (alkyl or aryl SO.sub.3.sup..crclbar.); or from C.sub.1-4 aliphatic carboxylic acids such as formic acid, acetic acid or propionic acid (C.sub.1-3 alkyl CO.sub.2.sup..crclbar.). Double salts of the dye salts obtained according to the invention can often be advantageously produced, especially the zinc chloride double salts. In this case, X.sup..crclbar. represents such complex anions as the trichlorozincate ion (ZnCl.sub.3.sup..crclbar.).